PROJECT SUMMARY The gastrointestinal tract is composed of a single layer of epithelial cells that separates the inside of the body from a complex and vast community of harmless micro-organisms (the microbiome) that live in the intestine and provide essential functions to the host. Underneath the epithelial lining is the largest population of immune cells in the mammalian body that serves as a critical monitor of these micro-organisms sensing the presence of potential disease-causing pathogens. Inflammatory bowel disease (IBD) is thought to result when the immune system inappropriately responds to the microbiome with an aggressive attack resulting in intestinal inflammation and damage. The immune signals that recognize and respond to bacterial and viral components of the microbiome are incompletely understood. Interferons (IFN) play a critical role in this process with diverse activities but are most known for their inherent anti-viral activities. IFNs are divided into three classes: type I IFNs (that comprise IFN?, ? and others), type II IFNs (IFN?) and the more recently identified type III IFNs or IFN?s. Most studies investigating the relationship of IFNs and IBD have focused on the relevance of type I IFNs. The cellular targets of IFN?s are much more limited and defined, compared to those of type I IFNs, because the specific receptor for IFN?, IFNLR1, is almost exclusively expressed in epithelial cells. We have recently demonstrated that IFNLR1 is also expressed on neutrophils. More importantly, we have demonstrated in mice that: 1) IFN? targets neutrophils; 2) IFN? blockade enhances inflammation; 3) commensal enteric viruses regulate IFN?-dependent responses; and 4) exogenous IFN? administration suppresses intestinal inflammation. Further support of a critical role for IFN? in mucosal homeostasis beyond murine biology comes from our recent identification of an infant with very early onset IBD (VEOIBD) that has homozygous damaging mutations in two IFN?s. Collectively, these findings serve the basis of our hypothesis that IFN? is a critical modulator of mucosal homoeostasis in mice and humans and may be an attractive therapeutic agent for IBD. In this project, we will benefit from the joint expertise of two co-principal investigators (Snapper and Zanoni) and capitalize on a vast compilation of conditionally targeted murine models, deep expertise in murine models of IBD, and relevant patient-derived materials. We propose four aims where we plan: 1) to assess the protective and therapeutic role of IFN? in acute and chronic models of colitis; 2) to determine the cell population(s) that mediates IFN?- dependent colitis protection and therapeutic responses; 3) to assess the role of microbiota in IFN?-dependent regulation of intestinal inflammation and IFN?-dependent therapeutic responses; 4) to assess how mutations in human IFN? alter mucosal homeostasis. Completion of the aims of this grant will aid in defining the precise mechanisms by which IFN?s protect against IBD in mice and humans and will explore whether IFN?s can be used as a therapeutic agent for the treatment of IBD.